Alfred Hitchcock: in the first moments of the film in the crowd - he is the only one not applauding the speaker. He can also be seen in a following scene, after the body in the river has been found, standing next to a couple discussing Jack the Ripper.
Alfred Hitchcock's shooting schedule allowed filming to begin at 8 am and finish at 6 pm every day whilst on location in Covent Garden in London. One day during filming, Hitchcock was in the middle of finishing a take when a union representative showed up to inform him that it was 6:15 pm and that they had to stop filming. Hitchcock became furious and threatened to walk off the set and film "Frenzy" back in Hollywood. After that, no more union representatives were allowed on the set.
The film and its source book ("Goodbye Piccadilly, Farewell Leicester Square") were inspired by the real-life unsolved crimes of the serial killer known as "Jack the Stripper". Unlike in the story, the real killings (which terrified London in the early 1960s) mirrored elements of "Jack the Ripper", in that the killer attacked prostitutes and that the killings mysteriously stopped.
The role of Robert Rusk was originally offered to Michael Caine. After Caine declined the role he later mentioned in his memoirs how Hitchcock completely ignored him when they met in a hotel a few years later.
This was the first Alfred Hitchcock film to show nude scenes, three in total. Barbara Leigh-Hunt (rape of Mrs. Blaney) and Anna Massey (after she's spent the night in the hotel with Blaney) each had nude models as stand-ins. The third nude scene is a last blonde victim.
Henry Mancini was originally hired to score the film. According to accounts, upon hearing the proposed score, Alfred Hitchcock yelled at Mancini: "If I had wanted Bernard Herrmann, I would have hired him!" Mancini was fired from the project. His recording of his main title to Frenzy is available on one of his compilation of film music excerpts.
The market scenes are set in Covent Garden, London, when Covent Garden was an actual fruit and flower market. This closed in 1974 and moved to Nine Elms and the old market was slated for demolition. Most of it, including the central halls were saved and now is a listed and popular tourist destination.
Several of the cast were unhappy with the lack of authenticity and Britishness of some of the dialogue. Jon Finch used to send notes to Alfred Hitchcock's secretary with suggested improvements. Hitchcock was not always pleased at this: "Jon, I said you could make alterations. I didn't say you could rewrite the whole script." However many of Finch's script amendments were indeed used in the final film.
Midway through the film, there is a famous continuous shot in which the camera backs away from the door of Rusk's upper-floor apartment and descends the staircase, seemingly without a cut, to the ground level, out the building's front door, and then to the opposite side of the street. The interiors were shot with an overhead track in a studio, and there is an imperceptible cut as a man passes by the front door, carrying a sack of potatoes. This is subtly blended into a new shot of the camera pulling away from the building exterior that was actually used on location.
The bags of pork scratchings displayed in the bar owned by Felix Forsythe (Bernard Cribbins) show the prices in dual currency (1/- or 5p). The same is true on various price lists seen in the market. While the UK had introduced decimal currency in February 1971, both the new and old currencies were used until around 1973; hence in some places, you would have found dual currency in use and on display.
Alfred Hitchcock originally planned to do his cameo as the body floating in the river. A dummy was even constructed to do the shot. The plans were changed and a female body, a victim of the Necktie Murderer, was used instead. Hitchcock instead became one of the members of the crowd who are listening to the speaker on the river bank. The dummy of Hitchcock was used in the typically humorous trailer hosted by Hitchock himself.
One of the awful meals prepared for Chief Inspector Oxford by his wife -- cailles aux raisins (quail with grapes) -- is what both Andre and Wally order at the beginning of the film My Dinner With Andre.